1. Technical Field of the Invention
The present invention relates generally to control apparatuses for automotive alternators and electric power generation systems for vehicles. More particularly, the invention relates to a control apparatus for an automotive alternator which has a capability of reliably detecting disconnection between the alternator and an on-board battery to be charged by the alternator.
The disconnection includes a state where a conducting wire electrically connecting the alternator and the battery is broken and a state where the conducting wire is detached from at least one of terminals of the alternator and the battery. This definition will be applied to all the description in this specification.
2. Description of the Related Art
Japanese Patent First Publication No. H06-276691 discloses a control apparatus for an automotive alternator, which includes a disconnection detector for detecting disconnection between the alternator and an on-board battery to be charged by the alternator.
In this control apparatus, both an output voltage of the alternator and a terminal voltage of the battery are detected. The disconnection between the alternator and the battery is detected based on the characteristic that the output voltage of the alternator is constant regardless of the disconnection whereas the terminal voltage of the battery decreases with occurrence of the disconnection. More specifically, in the control apparatus, the disconnection is detected when the terminal voltage of the battery is lower than a reference voltage.
However, since both the output voltage of the alternator and the terminal voltage of the battery have to be detected, the number of terminals and leads required for the disconnection detection increases. Consequently, the control apparatus and thus the entire electric power generation system become not only complex but also expensive.
U.S. Pat. No. 7,009,366 discloses a voltage regulator for an automotive alternator, which includes a disconnection detecting circuit for detecting disconnection between the alternator and an on-board battery to be charged by the alternator.
In this voltage regulator, only an output voltage of the alternator is detected. The disconnection between the alternator and the battery is detected based on the characteristic that upon being energized, the output voltage of the alternator with the disconnection is higher than that without the disconnection. More specifically, in the voltage regulator, the disconnection is detected when a voltage difference exceeds a predetermined threshold; the voltage difference is the difference between the output voltage of the alternator detected when a field transistor is turned off and that detected when the field transistor is turned on.
However, in case that the battery has been fully charged, almost no current flows from the alternator to the battery even without the disconnection. In other words, in such case, the difference in the output voltage of the alternator between with and without the disconnection is very small. Consequently, it becomes difficult to reliably detect the disconnection.
Japanese Patent First Publication No. H07-15886 discloses a control apparatus for an automotive alternator, which includes a warning circuit for detecting and warning of disconnection between the alternator and an on-board battery to be charged by the alternator.
In this control apparatus, a voltage at a feed terminal of an alarm lamp is detected; the feed terminal is electrically connected to a plus terminal of the battery. The disconnection between the alternator and the battery is detected based on the characteristic that the voltage at the plus terminal of the battery and thus that at the feed terminal of the alarm lamp decrease with occurrence of the disconnection. More specifically, in the control apparatus, the disconnection is detected when the voltage at the feed terminal of the alarm lamp drops below a predetermined level.
However, with the above configuration, the alarm lamp must be supplied with electric power from the battery. Thus, the control apparatus cannot be applied to cases where the alarm lamp is supplied with electric power from a power source other than the battery.